Tensile Fracture of Welded Polymer Interfaces: Miscibility, Entanglements and Crazing

Large-scale molecular simulations are performed to investigate tensile failure of polymer interfaces as a function of welding time $t$. Changes in the tensile stress, mode of failure and interfacial fracture energy $G_I$ are correlated to changes in the interfacial entanglements as determined from Primitive Path Analysis. Bulk polymers fail through craze formation, followed by craze breakdown through chain scission. At small $t$ welded interfaces are not strong enough to support craze formation and fail at small strains through chain pullout at the interface. Once chains have formed an average of about one entanglement across the interface, a stable craze is formed throughout the sample. The failure stress of the craze rises with welding time and the mode of craze breakdown changes from chain pullout to chain scission as the interface approaches bulk strength. The interfacial fracture energy $G_I$ is calculated by coupling the simulation results to a continuum fracture mechanics model. As in experiment, $G_I$ increases as $t^{1/2}$ before saturating at the average bulk fracture energy $G_b$. As in previous simulations of shear strength, saturation coincides with the recovery of the bulk entanglement density. Before saturation, $G_I$ is proportional to the areal density of interfacial entanglements. Immiscibiltiy limits interdiffusion and thus suppresses entanglements at the interface. Even small degrees of immisciblity reduce interfacial entanglements enough that failure occurs by chain pullout and $G_I \ll G_b$

Multiscale simulations of graphite-capped polyethylene melts: brownian dynamics/kinetic Monte Carlo compared to atomistic calculations and experiment

We propose a multiscale simulation strategy capable of investigating and predicting the interfacial structure and dynamics of polymer/solid interfaces over a wide range of time and length scales. The lowest level of description involves atomistic molecular dynamics simulations providing useful information for the parametrization of the higher level of description, a mesoscopic hybrid particle/field slip-spring model [Macromolecules 2017, 50, 3004, 4524]. The mesoscopic model is extended so as to capture the interfacial properties of polymer/solid interfaces observed in atomistic simulations and to make predictions for long polymer chains exhibiting relaxation phenomena over times inaccessible by conventional atomistic simulations. As a case study, our multiscale approach is applied to polyethylene/graphite interfaces; our approach is nevertheless generic and can be implemented for various other combinations of polymers and solid surfaces. The conformational properties of the mesoscopic chains in the vicinity of the interface are resolved through the incorporation of springs with variable, wall-distance-dependent stiffness. The interfacial kinetics is described in terms of adsorption and desorption processes of the mesoscopic beads, which are classified as adsorbed and free in relation to solid substrates; these processes are tracked by the kinetic Monte Carlo in the course of the mesoscopic simulation, with the relevant rate constants derived by hazard plot analysis of the atomistic molecular dynamics. For the classification of the polymer segments we develop a new mesoscopic definition for the adsorbed state, taking into account the position, energy, and previous history of the polymer beads, which yields results that are insensitive to the time-step of the simulation. The residence time of the macromolecules in the adsorbed state is quantified through various approaches (rate constants from cumulative hazard slopes and monitoring of detachment times); overall, the characteristic detachment time of long polymer chains adsorbed from the melt is found to increase with molar mass, M, as ∼M2.5 while the relaxation time from the autocorrelation of the end-to-end vector increases as ∼M3.5 the latter scaling has been observed for bulk samples also. The impact of the time-step and of diffusion-driven recrossings between adsorbed and free states of beads is assessed through an analytical solution to a simple model for "adsorption" near repulsive walls